The invention is based on a device for measuring the mass of a flowing medium, or flow rate meter. One such device is already known German offenlegungsschrift DE-OS 44 07 209; U.S. application Ser. No. 08/545,583, filed Nov. 3, 1995; which has a temperature-dependent measuring element accommodated in a measuring conduit.
The measuring conduit extends in the device from an inlet to an outlet, which is adjoined by an S-shaped deflection conduit. The deflection conduit is composed of a first segment and a second segment. The first segment has a rectangular corner and merges at a peripheral face with the second segment. The flowing medium flows from the outlet of the measuring conduit first into the first segment of the deflection conduit; this segment has a larger cross section than the measuring conduit, so that an abrupt flow transition exists, in the form of a step to the first segment. Next, the medium from the first segment of the measuring conduit is deflected by the corner into the transversely adjoining second segment of the deflection conduit and leaves that segment through an outlet opening, and then mixes again with the medium flowing past the device. The measuring conduit has side walls that are embodied in inclined fashion, so that the measuring conduit tapers toward the measuring element.
In an internal combustion engine, the opening and closing of the inlet valves of the various cylinders cause major fluctuations or pulsations in the flow, whose magnitude depends on the aspiration frequency of the individual pistons or on the engine rpm. The pulsations in the flow are propagated from the inlet valves via the intake line to the measuring element in the measuring conduit and onward from there. The effect of the pulsations is that depending on their magnitude, the thermal inertia and orientational insensitivity of the measuring element cause it to produce a measurement outcome that can deviate considerably from the flow velocity prevailing on average in the measuring conduit and the aspirated air mass of the engine that can be calculated from this velocity. The measuring conduit and the deflection conduit are adapted to one another in their dimensions such that if there is a pulsating flow in the intake line, the incorrect indication of the measuring element that occurs because of the flow fluctuations is minimal. Nevertheless, at high pulsation frequencies and a significant pulsation amplitude, fluidic-acoustical processes in the deflection conduit can still cause an incorrect indication of the aspirated air mass. This incorrect indication comes about particularly because downstream of the step, or tearing edge, from the measuring conduit to the deflection conduit major eddies can arise, whose rotational impulse varies only insignificantly when there is a pulsating oncoming flow. Hence there is a major difference in the relative velocities at the edge of the eddy and downstream of the measuring element, so that severe shear flows are present at the boundary face that are the cause of the generation of acoustical waves (sound waves) in the deflection conduit and the measuring conduit. These acoustical waves can interfere with the measurement signal of the measuring element, resulting in a minimum indication.